Harmonics and Reactive Power Compensation in Practice

8/13/2019 Harmonics and Reactive Power Compensation in Practice

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EN-TH04-11/2004 1 (21) Martti Tuomainen

Nokian Capacitors Ltd. P.O. Box 4 Tampere Finland Tel: +358-3-388311 Fax: +358-3-3883360

Harmonics and Reactive Power

Compensation in Practice

1. General

Harmonics in utility and industrial networks have an increasing trend allover the world. This is clearly related to the increasing use of non-linearloads and devices in industry and in commercial buildings. These non-linear devices are often thyristor or diode rectifiers, which thus contributeto the deterioration of the power quality in the networks, can be found forexample in the following applications:

- in variable speed drives (VSD)- for manufacturing and process industry- for inductive heating in metal industry- for lifts and air-condition pumps and fans in commercial

buildings

- in uninteruptable power supplies (UPS) for computers and otheressential loads in commercial and industrial buildings

- in computers and in other office equipment

In figure 1 there is a typical DC-drive with 6-pulse thyristor rectifier and infigure 2 a typical voltage source inverter drive with 6-pulse diode rectifier.Same rectifiers can be found also in uninterruptible power supplies (UPS).

MM

DC-

motor AC-motor

6-pulse diode

rectifierInverter

6-pulse thyristor

rectifier

L1

L2

L3

L1

L2

L3

Figure 1. Typical DC-drive Figure 2. Typical voltage source inverterwith 6-pulse thyristor rectifier. drive with 6-pulse diode rectifier.


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L

N

Figure 3. One phase rectifier used for Switch Mode Power Supplies.

In figure 3 there is one phase rectifier with capacitive smoothing used forSwitch Mode Power Supplies. This kind of power supplies are widely usedin computers, monitors and in many other electronic equipment.

Rectifiers produce harmonic currents having following harmonic orders orfrequencies:

n =fnff

= k p 1

where:

fn= frequency of the harmonic current

ff= fundamental frequency of the systemn = order of the harmonick = 1,2,3,.p = pulse number of the rectifier

If the rectifier is connected into an unlimited bus the amplitudes of theharmonic currents can be calculated as follows:

In=I1

n

(2)

where:

In= amplitude of the harmonic current order nI1= fundamental current of the rectifiern = order of the harmonic

However in real networks harmonic currents can have significantly higheramplitudes than calculated with formula (2) above. In next chapter thereare some measured harmonic currents of different kind of rectifiers.


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1.1 Harmonic currents in real networks

In figure 4 the are measured AC-side fundamental and harmonic currentsof a DC-drive with its load information. As can be seen the 5 thharmonic isin this case 28% corresponding to 632A of the fundamental whereas itstheoretical value according to formula 2 is 20%.

Supply current

A

2261

632

78

147

41

100

28

4

7

2

1

5

7

11

13

Order

of

harmonic

2.5MVA

5.4%

440V

Point of

measur.

11.4kV 60Hz

DC-drive

%P = 710kW

Q = 1525kvar

S1= 1682kVA

PF = 0.407

STOT= 1744kVA

- 0 . 7

- 0 . 5

- 0 . 3

- 0 . 1

0 . 1

0 . 3

0 . 5

0 . 7

0 3 . 3 3 6 . 6 6 9 . 9 9 1 3 . 3 2 1 6 . 6 5

Figure 4. Fundamental and harmonic currents of a DC-drive with high load.

1255

515

122

108

28

100

41

10

9

2

1

5

7

11

13

-100

100

0 0 0 0 0 0 0

P = 450kW

Q = 845kvar

S1= 957kVA

PF = 0.426

STOT= 1056kVA

2.5MVA

5.4%

440V

Point of

measur.

11.4kV 60Hz

DC-drive

Supply current

A

Order

of

harmonic %

Figure 5. Fundamental and harmonic currents of a DC-drive with low load.

In figure 5 there is the same DC-drive than in figure 4 but now with lower

load. Fundamental current is decreased from 2261A to 1255A. Howeverthe percentages of the harmonics are clearly increased. For example


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It should be noted that zero sequence harmonics accumulates in theneural wire. These zero sequence currents in neutral wire may reachvalues bigger than currents in each phase.

2. Effects of Harmonics

2.1 Transformers

Harmonic currents cause an increase in copper losses and stray fluxlosses. Harmonic voltages cause an increase in iron losses. The overalleffect is a higher temperature rise, as compared to purely fundamentalsinusoidal current and voltage operation. It shall be noted that theseadditional losses due to harmonics will rise in proportion to the square ofthe current and frequency, resulting in decreased fundamental loading

capacity of the transformer. When selecting correct rated power for thetransformer to supply non-linear loads, an adequate derating should bemade to ensure that the temperature rise of the transformer will remainwithin permissible limits. Also it should be kept in mind that all additionallosses due to the harmonics will be paid by the customers in terms ofkilowatt hours consumed. Harmonics applied to the transformer can alsolead to increased audible noise.

2.2 Power Cables

Non-sinusodial currents in conductors will cause more heating than that

what would be expected for the RMS value of the waveform. Thisadditional heating is caused by two phenomena known as skin effect andproximity effect, both of which depend on frequency as well as conductorsize and spacing. These two effects result as an increased ac resistance,which in turn leads to increased I

2x RAClosses.

2.3 Motors and Generators

The main effect of the harmonic currents and voltages in rotating inductionand synchronous machinery is increased heating caused by the iron andcopper losses at harmonic frequencies. These additional losses lead to

decreased machine efficiency and can also affect the torque developed.Pulsating torque output can affect product quality in cases where motorloads are sensitive to such variations. As examples from sensitive loadssome synthetic fibre spinning and some metal working applications can bementioned.

Also in case of rotating machinery, harmon ics can inc rease audible no iseemission as compared with sinusoidal magnetisation. Harmonic pairs like5

thand 7

thcan create mechanical oscillations at 6

thharmonic frequency in

a generator or in a motor-load system. Mechanical oscillations are causedby oscillating torques due to the harmonic currents and the fundamentalfrequency magnetic field. If the mechanical resonance


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frequency coincides with the electrical stimulus frequency, highmechanical forces can be developed and there is a risk of mechanicaldamages in the machinery.

2.4 Electronic Equipment

Power electronic equipment is sensitive to harmonic distortion of thesupply voltage. This equipment is often synchronising its operation to thevoltage zero crossings or to other aspects of the voltage wave shape.Harmonic distortion of the voltage can lead to the shifting of the voltagezero crossing or change the point where one phase to phase voltagebecomes higher than an other phase to phase voltage. Both of these areimportant points for different kind of power electronic circuit controls.Misinterpretation of these points by the control systems can lead to themalfunction of the control system. Disturbances of the telecommunication

equipment are also possible due to the inductive or capacitive couplingbetween power and telecommunication lines.

Computers and some other kind of electronic equipment likeprogrammable controllers require usually that the total harmonic voltagedistortion (THD) of the supply is less than 5% and one individual harmoniccomponent is less than 3% of the fundamental voltage. Higher distortionvalues may result in misoperation of the control equipment, which in turncan lead to production and process interruptions, which can have higheconomical consequences.

2.5 Switchgear and Relaying

Like in other type of equipment too harmonic currents cause also inswitchgear additional losses leading to increased heating and reducedfundamental current carrying capability. An increased temperature ofsome insulating components results in shortening of their lifetime.

Older solid-state tripping devices on low voltage circuit breakers haveresponded to the peak currents. This type of tripping devices may causenuisance tripping in feeders supplying non-linear loads. New trippingdevices respond to the RMS values of the current.

The response of the protective relays to the distortion depends a lot onthe measuring principle used and there is no any common rules whichcould be used to describe what is the impact of harmonics on large varietyof the relays. However it can be said that normal harmonic distortionlevels in networks do not cause problems in relay operation.

2.6 Power Factor Correction Capacitors

Capacitors differ from other t ype of equipment due to its capacitive nature,which can dramatically change the system impedance under


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system resonant condition. The reactance of a capacitor bank decreaseswith the higher frequencies, and therefore, bank acts as a sink for higherharmonic currents. This effect increases the heating and dielectric stressof the insulation material. Frequent switching of non-linear magneticcomponents like transformers can produce harmonic currents, which willadd to the loading of the capacitors. It should be noted that the fusesusually do not provide overload protection for the capacitors. The result ofthe increased heating and voltage stress due to harmonics is a shortenedlifetime.

The major concern arising from the use of capacitors in power systems isthe possibility of the system resonance. This effect leads to harmonicvoltages and currents that are considerably higher than they would be inthe case without resonance.

2.6.1 Harmonics and Parallel Resonance

Harmonic currents produced for example by variable speed drives can beamplified up to 10-15 times in parallel resonance circuit formed by thecapacitance of the capacitor bank and the network inductance.

Amplified harmonic currents through the capaci tors can lead to internalover heating of the capacitor unit. Please note that currents having higherfrequency than fundamental frequency cause more losses than 50Hzcurrent having same amplitude. An example of parallel resonance circuitwith its equivalent circuit is shown in figure 8.

11kV 150MVA 50Hz

Harmonics

VariableSpeed

Drive

Capacitorbank

400V

Harmonic source

Networkinductance

Capacitance

of the

capacitor

bank

Figure 8. Parallel resonance circuit and its equivalent circuit.

2.6.2 Harmonics and Series Resonance

In case the voltage of the upstream network is distorted the seriesresonance circuit, formed by capacitance of the capacitor bank and theshort circuit inductance of the supplying transformer, can draw high

harmonic currents through the capacitors. Series resonance can lead to


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high voltage distortion levels at low voltage side of the transformer. For anexample of series resonance circuit please refer to figure 9.

400V

Harmonics

Capacitor

bank

Transformers

Inductance's

11kV 150MVA 50Hz

Harmonics

Variable

Speed

Drive

400V

Harmonic

source

Capacitanceof the capacitor

bank

11kV

Figure 9. Series resonance circuit and its equivalent circuit.

2.6.3 Recommendations.

Whenever there is unlinear load (dc-drives, inverters, UPS, all kind ofrectifier, etc.) connected to the bus at which capacitor bank is intended tobe used care should be taken when designing reactive power

compensation system.

To avoid parallel or series resonance at low voltage level filter or blockingtype capacitor banks should be used.

In cases where there are harmonic limits imposed by the utility orgenerating authorities quite often filter capacitor bank turns out to benecessary to meet requirements like stated for example in IEEE standard519-1992 or in Engineering Recommendation G5/3. For typical filtercapacitor bank arrangements where there are 3 tuned branches for 5

th, 7

th

and 11th

harmonic please refer to figure 10. The number of tunedbranches depends on the harmonics to be absorbed and on requiredreactive power to be compensated. In some cases even one tuned branchmight be enough to reach desired voltage distortion and target powerfactor.

To be able to design filter capacitor bank harmonic producing load shouldbe identified and at existing plants harmonic measurement is desirable.


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5th 7th 11th

100%

80 -90%

10 - 20%

11kV 150MVA 50Hz

400V

Figure 10. Reactive power compensation with filter capacitor banks.

According to IEEE 519-1992 individua l vo ltage distor tion is allowed to be3% of the fundamental. If for example at some bus non-linear load hascaused an individual voltage distortion which has been measured to beless than 3% without capacitors, it may lead to assumption that any kind

of electrical equipment could be connected at the same bus without anyconcern. Please note however, that whenever capacitor bank without anyreactors is connected at such bus, there will be a certain parallel andseries resonance frequency. If this resonance frequency coincides withsome harmonic frequency, significant amplification of harmonic currentsand voltage distortion will take place.

In cases where there are no harmonic limits blocking type capacitors canbe used. However it should be kept in mind that major part of theharmonics is then injected into the upstream network. For typical blockingtype capacitor bank please refer to fig 11. The required number of steps

depends on load power factor and target power factor. For the design ofblocking capacitor bank voltage distortion limits are usually given. Typicalvalues for low voltage could be for example: U3rd = 0,5% U5th = 5% andU7th = 5%.

Typical detuning frequencies are 204hz corresponding to 6% reactor and189hz corresponding to 7% reactor respectively. The use of 7% reactorsallows normally more non-linear load to be connected than the use of 6%reactors. The linearity of the iron core of the reactor should be designedso that saturation is not possible with inrush current and at rated voltagedistortion.


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100%

10 -30%

70 - 90%

11kV 150MVA 50Hz

400V

7% 7% 7%

Tuning frequency 189Hz

Figure 11. Reactive power compensation with blocking capacitor banks.

When designing reactive power compensation system, for example for anew commercial building, the use the capacitor units having higher ratedvoltage than system voltage (for example 525V units in 400V system) isquite often justified if it is not known what kind of load will come with the

tenants. The use of higher rated units enables reactors to be added lateron if it turns out that load will produce harmonics.

Whenever there is a doubt that ambient temperature of the capacitor unitsmay exceed upper limit of their temperature category it is advisable to usecooling fans in capacitor cubicles. Please note also that in cases whereblocking or filter reactors are used forced cooling should always be usedsince the reactors are causing a lot more losses than capacitor units.

3. Documented Power System Harmonic Resonance Casesand Problem Solutions

3.1 Case 1

In a rather big office building many capacitor units were reported to bethermally damaged. Failed units were found in automatically controlledcapacitor banks connected on transformers supplying UPS equipment forcomputers. Harmonic measurements were made to find out reason fordamages. In figure 12 there are measured fundamental and harmoniccurrents of the supplying transformer as well as measured total harmonic


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distortion (T.H.D.) of the voltage. As can be seen there is heavyresonance at 11

thharmonic when two 50kvar steps are on causing the 11

th

harmonic current of 30A (produced by the UPS) to be amplified to 283Acorresponding to an amplification factor of approximately 10. Please notealso that T.H.D. of the voltage has increased to 19.6%.

10kV 50Hz

800kVA

zk= 5%

400V

Point of

measurem.

- 5 x 50kvar -

UPS

12-pulse

Supply currentOrder

of

harmonic 0 kvar 50kvar 100kvar

A A A

1 600 538 512

5 10 8 11

7 4 5 9

11 30 62 283

13 6 9 44

17 2 8 2

THD(V) 3.8% 5.9% 19.6%

Figure 12. Measured supply current in case 1. Capacitors withoutreactors.

In fig 13 there are measurement results of the capacitor bank current. TheRMS current of the capacitor bank was 364A with two steps on

corresponding to 2.5 times rated current thus the reason for damages wasrevealed. According to IEC 60831-1 (Standard for Low VoltageCapacitors) the permissible current for capacitors is 1.3 times ratedcurrent.

10kV 50Hz

800kVA

zk= 5%

400V

Point of

measurem.

- 5 x 50kvar -

UPS

12-pulse

Current of the

capacitor bank

50kvar on

A

100kvar on

A

Order

of

harm.

1 70 142 5 4 11

7 2 4

11 36 327

13 16 69

17 9 3

IRMS/A 83 364

Figure 13. Measured capacitor bank current without reactors for case 1.


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Since the harmonic measurements confirmed that there has been aresonance in the power system the reactive power compensation systemwas redesigned and the decision was made to use blocking capacitorbanks with 7% reactors. In fig 14 there are measurement results of thesupply current and the THD of the voltage when the new capacitor bankwith 7% reactors was commissioned.

Supply currentOrder

of

harmonic 0kvar 50kvar 100kvar

A A A

1 773 735 691

5 13 17 20

7 7 6 5

11 45 42 40

13 16 15 14

THD(V) 6.4% 6.0% 5.7%

10kV 50Hz

800kVA

zk= 5%

400V

Point of

measurem.

- 5 x 50kvar -

UPS

12-pulse

7% reactors

189Hz

Figure 14. Measured supply current in case 1 with new blocking capacitor bank.

Please note that with this new capacitor bank resonance can be avoidedwith any number of steps connected on thus there is no any amplificationof the harmonic currents. To verify the new design commissioning of the

new bank was made under maximum non-linear load condition and theharmonic currents were found to behave like expected.

In fig 15 there are measured currents of the new blocking capacitor bank.As can be seen from the curren ts remain within expec ted va lues wi th anynumbers of steps connected on.

Order

of

harmonic 50kvar 100kvar 150kvar A A A

1 75 151 227

5 5 10 13

7 1 1 1

11 3 6 8

13 1 2 3

RMS / A 75.2 51.5 227.6

Current of the

capacitor bank800kVA

zk= 5%

400V

Point of

measurem.

- 5 x 50kvar -

UPS

12-pulse 7% reactors

189Hz

Figure 15. Measured capacitor bank current with reactors for case 1.


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3.2 Case 2

The one line diagram in fig 16 is an extract from the whole supply systemof a plastic moulding company. The fixed 150kvar capacitor bank hadsuffered several failures but kept in service changing time to time failingunits. To find out reason for continuous failures a harmonic measurementwas carried out with the results as shown in fig 9. The RMS current of thecapacitor bank was measured to be 371A main harmonic component beingthe 11 thharmonic. Measured RMS current corresponds to the 1.71 timesrated current, which certainly explains why capacitors kept failing. Sincethe voltage T.H.D., even without capacitors, turned out to be as high as8.1%, the company is now considering to use filt er capacitor banks forreactive power compensation to be able to maintain good power quality forall equipment.

Order

of

harmonic

Capacitor

current

IC/ A

Voltage distortion

capacitor

%

With Without capacitor

%

1

5

7

11

209

100

45

283

-

9.7%

2.7%

11.3%

-

7.7%

1.3%

2.3%

371 15.1% 8.1% RMS

20kV 50Hz

1000kVA

zk= 6%

400V

Point of

measurem.

150kvar

Figure 16. Measured current of the capacitor bank in case 2.

3.3 Case 3

The owner of the company purchased the capacitor bank sown in one linediagram in figure 17. His decision was based on the electricity bill onlyaccording to which he was paying penalty for poor power factor.Altogether 400kvar were needed to improve power fac tor up to the pena ltylimit.The measurement during the commissioning of the capacitor bankrevealed that 500kVA transformer supplying the factory was slightlyoverloaded and 5 thharmonic current was 62A being 9% of thefundamental. When capacitor bank was connected on fundamental currentdecreased to 492A because reactive power was compensated but 5

th

harmonic current was amplified to 456A being now 93% of thefundamental and the THD of the voltage increased to 16.2% being totally

unacceptable for the supply of the load.


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Capacitor bank was switched off and its replacement with blocking typecapacitor bank was put on the way.

Supply current

Withoutcapacitors

With400kvar

A % A %

1

5

772

62

100

9

492

456

100

93

RMS

current 774 100.4 671 136

Order of

harmonic

20kV 50Hz

500kVA

In= 722

400V

Point of

measurem.

- 8 x 50kvar -100.4

THD

voltage 16.25.6

Figure 17. Measured supply current for case 3.

3.4 Case 4

In this case measurements were made to determine what kind of reactivepower compensation system would be needed to improve power factor upto penalty limit. As can be seen from the measurement results the voltagewas heavily distorted and the voltage THD was measured to be 12%.

It was evident that capacitor bank without reactors could not be used. Dueto the high voltage distortion the decision was made to use filter capacitorbanks for reactive power compensation.

Supply current

Filtersoff

A

5thon

A

5th,7thon

A

Allon

A

1387

402

44

245

1065

51

57

193

956

51

1

77

860

52

2

10

THD(V) 12.0% 5.4% 3.0% 2.0%

1

5

7

11

Orderof

harm.

5th 7th 11th

20kV 50Hz

400V

2MVA

zk=6%

Point ofmeasurem.

Figure 18. Measured supply current and voltage THD for case 4.


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In fig 18 there are measurement results which were obtained during thecommissioning of the filter capacitor banks. When all fil ters wereconnected on the voltage THD decreased from 12% to 2% only which isconsidered to be very good value for a low voltage supply system. Pleasenote also a significant decrease of the fundamental supply current byapproximately 520A due to compensated reactive power. Also harmoniccurrents have been absorbed effectively and supply current met thespecified harmonic limits.

3.5 Case 5

In fig 19 there is an extract from a big paper mill supply system with a10Mvar 20kV capacitor bank which was suffering from time to timenuisance tripping by the over current relay. Harmonic measurement

showed abnormal high voltage distortion of 10.8% at 20kV bus and the 5th

harmonic current in the supply was 135A.

Supply current

10.8%

1

5

435

135

355

38

100 100

1.5%THD(V)

A A% %

Without

Capacitor

With

Capacitor

Order

of

harmonic

6 1.4

10Mvar

Capacitor

bank

20kV

110kV

Point of

measurem.

20MVA

zk=10%

Figure 19. Measured supply current and voltage THD for case 5.

When capacitor bank was switched off the voltage distortion decreased to1.2% and 5

thharmonic current in the supply was 6A only. In this medium

voltage resonance case amplification factor reached a high value ofapproximately 22. This capacitor bank was redesigned taking into accountharmonic currents produced by the DC-drives of the paper machine. Aftercomputer simulations with several possible network configurations, the 5 th

harmonic filter turned out to be the best solution.


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Modification of the capacitor bank was made by increasing its ratedvoltage by adding one more capacitor unit in series with existing units andinstalling air core filter reactors.

3.6 Case 6

The medium voltage networks of the utilities are considered to have areasonably low voltage distortion. However in cases utilities are usingsubstation capacitor banks without blocking reactors there is always thepossibility of resonance if substation is supplying industrial plants havingharmonic producing load.

Figure 20. Voltage Curve Form at a Utility 11kV Substation and its Harmonic content

In fig 20 there is measured voltage curve form at 11kV bus of a substation

where installed capacitor banks did not incorporate blocking reactors. Ascan be seen voltage is heavily distorted due to the resonance and 5

th

harmonic voltage component was measured to be 22.2% of thefundamental. If this voltage is supplying a MV / LV transformer wherethere are capacitors on low voltage the series resonance circuit formed bycapacitance of the capacitor bank and the short circuit inductance of thesupplying transformer can draw high harmonic currents through thecapacitors.

3.7 Case 7

In fig 21 there are measured voltage THD values over a period of 20 hoursat a 11.4kV utility bus supplying several small and medium size industries.It was evident that utility capacitor banks caused the harmonics producedby the non-linear loads at the factories to be amplified. The harmonicmeasurements were initiated due to the information that at one plantinstalled LV side filter capacitor banks were suffering time to timenuisance tripping. Harmonic analysis revealed that the distortion wasmainly consisting of 5

thharmonic with a maximum value of 8.1% during

the measurement period exceeding clearly the 3% limit stated by the

utility.


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Calculations were made using measured distortion values and theyresulted in RMS currents in LV side filters, which clearly exceeded settingof the thermal over current relay of the 5

thfilter. If action is not taken to

remove the resonance in the 11.4kV system filters at LV should bemodified to be blocking capacitor banks with 6% or 7% reactors which willresult in higher harmonic currents to the utility system contributing to thefurther deterioration of the 11.4kV power quality.

VOLTAGE THD AT 11.4kV (%)

0,0

2,0

4,0

6,0

8,0

10,0

12,0

17.24 19.24 21,24 23.24 1,24 3,24 5,24 7,24 9,24 11,24

TIME

Figure 21. Measured Voltage THD of a Utility 11.4kV System for Case 7

3.8 Case 8

In fig 22 there is the one line diagram of a utility substation supplying 7factories. On transformers TR1 -TR6 the load consists partly of non-linearloads but on transformer TR7 there is normal AC load only. Reactivepower compensation was made using automatically controlled capacitor

banks without any reactors. The manufacturer of the capacitor banks wasinformed that several capacitor units and fuse bases were thermallydamaged on some transformers. A harmonic measurement was carried outand voltage distortion values on some transformers are shown in fig 15.Please note that TR7 is also suffering poor power quality due to the 5%distortion on 20kV utility bus.

While redesigning the reactive power compensation systems for thefactories, the decision was made that harmonic currents shall be absorbedon the transformers they are produced thus harmonic filters should beused. The reactive power, number of branches and tuning frequencies of

the filter capacitor banks were designed according to load


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on each transformer. Of course there was no need to replace existingcapacitor bank on transformer TR7 because there was linear load only.

TR1 TR2 TR3 TR4

TR5 TR6 TR7

20kV THD 5.0%

THD 13.6%

THD 7.9% THD 16.7% THD 7.1%

10MVA

110kV 50Hz

Figure 22. One line diagram for case 8 with measured voltage THDvalues. Capacitor banks without reactors

In figure 23 there is the one line diagram after existing capacitor banks ontransformers TR1 -TR6 were replaced by the filt er capacitor banks. As canbe seen the measured distortion values remains within very low valuessince harmonics are effectively absorbed there were they are produced.

Please note also that the power quality on transformer TR7 is now wellwithin the limits since filters on LV side of the transformers TR1 -TR6 havereduced harmonic emission to the 20kV network decreasing voltagedistortion from 5% to 0.8%.

This case clearly demonstrates the important role of the utility inmaintaining sufficient power quality for all customers.


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5th,7th,11th 5th,7th,11th 5th,7th,11th 5th,7th,11th

5th,7th,11th 5th,7th,11th,13th

TR1 TR2 TR3 TR4

TR5 TR6 TR7

20kV THD 0.8%

THD 2.2%

THD 1.7% THD 1.7% THD 1.5%

10MVA

110kV 50Hz

Figure 23. One line diagram for case 8 with measured voltage THD valueswith filter capacitor banks.

3.9 Case 9

This case deals with a series resonance circuit formed by a processtransformer and a 300kvar 660V capacitor bank. In figure 24 there is anextract from a big paper mill supply system with 6kV harmonic filters tunedfor 5 th, 7 thand 11 thharmonic. A voltage distortion measurement was made

at LV side of one of the 12 process transformers having a fixed 300kvarcapacitor bank. According to measurement results voltage distortion was1.94% only when 6kV filters were on but increased to 7.15% when filterswere switched off. The increase of the voltage distortion clearly verifiesthe series resonance and also the fact that harmonics can proliferatethrough MV network to the LV side of other transformers. In this kind ofseries resonance circuit the voltage distortion is caused in the short circuitimpedance of the transformer by the harmonic currents flowing from MVnetwork through to the transformer in to the capacitor bank.


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110kV 50Hz

6.0kV

--12 pcs--

660V

450kvar 300kvar

5th 7th 11th

Filter Filter Filter

DC-Drives

THD(V)

at 660V

WithFilters

WithoutFilters

1.94% 7.15%

Point of

measurement

2MVA

zk=6%

2MVA

zk=6%

Figure 24. Measurement results for case 9 with and without 6kV filters.

3.10 Case 10

In case 10 Nokian Capacitor was asked to design and deliver harmonicfilters for big chemical plant involving 18-pulse rectifier. According tocustomers specification 47.6Mvar at 22kV were needed to reach powerfactor limit imposed by the utility. The maximum total voltage distortionTHD(V) at 22kV bus was to be reduced to 3%. To reach required voltagedistortion four branch filter consisting of 5 th , 7 th, 11 thand 13 thfilters wasdesigned.

The total required reactive power of 47.6Mvar were divided between fourbranches assigning higher Mvar outputs to filters tuned for 11

thand 13

th

harmonics. This arrangement resulted in lower impedance of thesebranches, which in turn means better absorption of higher order harmonicslike 17 th, 19 th etc. In figure 25 there is the one line diagram of the systemwith measured voltage distortion at 22kV bus with and without filters.

As can be seen the total harmon ic vo ltage distor tion is reduced from10.9% without filters to 1.9% when filters are connected on thus therequired 3% distortion limit was clearly met.


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161kV 60Hz

100MVA

zk=10.5%

22kV

5th 7th 11th 13th

9.0Mvar 8.7Mvar 15.0Mvar 14.9Mvar

18-pulse

rectifier

Order

of

harm.

Filters

off

%

All filters

on

%

5 0.4 0.3

11 1.0 0.3

13 0.6 0.3

17 4.9 0.5

19 5.8 0.6

29 1.8 0.3

31 1.3 0.2

35 3.9 0.5

37 4.8 0.6

47 2.6 0.1

49 1.7 0.1

THD(V) 10.9 1.9

Figure 25. One line diagram for case 10 with measured voltage THDvalues without and with filter capacitor banks.

4. Conclusions

In most cases harmonic problems show up at the industrial plants or at thecommercial customers before distortion level on the utility network reachcritical values. Resonance conditions created by the use of the capacitorswithout reactors at the customers facilities will lead to high distortion onthe low voltage bus where the capacitors are connected on. Problems likemotor overheating, transformer heating and malfunction of the electronicequipment within the customers facilities are likely to occur. Therefore itis imperative for the customers to understand possible harmonic problemsand take care that harmonic distortion remains within reasonable limits.

With previous documented cases it has become clear that wheneverdesigning reactive power compensation systems harmonics should be keptin mind. Also it has been shown that there are solutions for the problemsarising from harmonics. With proper design resonance situations can beavoided and possible emission limits imposed by the utility can be met.

Computer calculations allow a quick look at different network conditionsand their output can be used as design data. However measurements atsites provide valuable information and their results can be used as inputvalues for computer simulations or they can verify calculation results.

Date post:	04-Jun-2018
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Harmonics and Reactive Power Compensation in Practice

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